2916
J . Org. C h e m . 1988,53, 2916-2920
72% isolated yield; [a]23D +25.2" (c 2, MeOH); 85% ee. Hydroboration of 2-Methyl-4,5-dihydrofuran w i t h 4dIcrzBH. The reaction (25-mmol scale) was done as described under cis-3-hexene at 0 "cfor 8 h. The trialkylborane ("B NMR 6 86.4) was oxidized and the reaction mixture worked as described above to afford 1.9 g of truns-2-methyl-3-hydroxytetrahydrofuran, (74% isolated yield): bp 91-92 "c (15 "); [.Iz3~ -29.91' ( C 2.5, MeOH); 70% ee. Hydroboration of 3,4-Dihydro-2H-pyran with 4-dIcrzBH. T o a stirred suspension of 4-dIcrzBH (25 "01, 7.2 g) in 15.5 mL of THF was added 4.6 mL (50 mmol) of 3,4-2H-dihydropyran a t 0 "C. The reaction mixture was stirred for 240 h a t 0 "C, methanolyzed (80% of 4-dIcrzBH reacted), oxidized, and worked
up as described under trans-3-hexene to afford 1.47 g of 3hydroxyktrahydropyran (58% isolated yield), bp 90 "C (20 mm). Preparative GC was done using columns c and d to furnish GCpure material: [.Iz3D +1.35" (neat), 11% ee.
Acknowledgment. We thank the National Institutes of Health (Grant GM 10937-24) and the D~~ Chemical
Company for support of this research program. We are grateful to Dr. Sukh Dev of Malti-Chem Research Center, Nandesari, India, and Dr. s, N. Mehra of Camphor 8c Allied Products Ltd., Clutterbuckgan, India, for generous gifts of (+)-2- and (+)-3-carene, respectively.
Chiral Synthesis via Organoboranes. 18. Selective Reductions. 43. Diisopinocampheylchloroborane as an Excellent Chiral Reducing Reagent for the Synthesis of Halo Alcohols of High Enantiomeric Purity. A Highly Enantioselective Synthesis of Both Optical Isomers of Tomoxetine, Fluoxetine, and Nisoxetine Morris Srebnik,' P. V. Ramachandran,2 and Herbert C. Brown* H. C. Brown and R. B. Wetherill Laboratories of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received December 8, 1987
Diisopinocampheylchloroborane, dIpc2BC1, reduces ring and chain substituted haloaralkyl ketones to the corresponding halo alcohols in excellent enantiomeric excess. In certain cases these alcohols can be upgraded by simple methods to essentially 100% ee. For instance, (+)- or (-)-3-chlorc-l-phenyl-l-propanol is initially obtained in 97% ee. Simple recrystallization then furnishes the pure enantiomers. These chiral halo alcohols are highly versatile intermediates. They can be readily cyclized to oxiranes and 2-substituted tetrahydrofurans with retention of chirality. Utilizing this methodology, we have developed an efficient, highly enantioselective synthesis of both optical isomers of the antidepressant drugs, Tomoxetine, Fluoxetine, and Nisoxetine, from the common intermediates (+)- or (-)-3-chloro-l-phenyl-l-propanol.
Wes5 and others6 have recently demonstrated the utility of chirally modified tri- and tetragonal boron compounds for the asymmetric reduction of prochiral ketones. In particular, we have shown that diisopinocampheylchloroborane, dIpc,BC1, (the symbol d indicates that the reagent is derived from (+)-a-pinene), enantioselectively reduces prochiral aralkyl ketones to the corresponding alcohols3 (eq 1). We have further discovered that certain a - t e r t -
alkyl ketones can be similarly chirally r e d ~ c e d .In ~ these initial investigations we restricted ourselves to relatively ~~
(1)Part of t h i s work was carried out at the Aldrich Chemical Company, Sheboygan Falls, WI. (2) Postdoctoral research associate on ARO Grant DAAG 29-82-K0047. (3) (a) Chandrasekharan,J.; Ramachandran, P. V.; Brown, H. C. J . Org. Chem. 1985, 50, 5446. (b) Brown, H. C.; Chandrasekharan, J.; Ramachandran, P. V. J. Am. Chem. SOC.1988,110, 1539. (4) Brown, H. C.; Chandrasekharan,J.; Ramachandran, P. V. J. Org. Chem. 1986,51, 3394. ( 5 ) Brown, H. C.; Park, W. S.; Cho, B. T. J. Org. Chem. 1986,51, 1934. (6) For leading references,see: (a) Imai, T.; Tamura, T.; Yamamuro, A.; Sato, T.; Wollman, T. A.; Kennedy, R. M.; Masamune, S. J . Am. Chem. SOC.1986, 108, 7402. (b) Itsuno, S.; Nakano, M.; Miyazaki, K.; Masuda, H.; Ita, K.; Hirao, A.; Nakahama, S. J. Chem. Soc., Perkin Trans. 1 1985,2039. (c) Soai, K.; Ovamada, H.; Yamanoi, T. J . Chem. SOC., Chem. Commun. 1984, 413. (d) Midland, M. M. In Asymmetric Synthesis;Morrison, J. D., Ed.; Academic: New York, 1983; Val. 2, p 45. (e) ApSimon, J. W.; Collier, T. L. Tetrahedron 1986, 42, 5157. (DSrebnik, M.; Ramachandran, P. V. Aldrichimica Acta 1987, 20, 9.
Table I. Asymmetric Reductions of Haloaralkyl Ketones with d I ~ ~ . B Cin1 T H F a t -25 "C -halo alcohol product" cyclized abs product % eeb config % ee 2-chloroacetophenone 96 Rd 96 2- bromoacetophenone Rd 86 86' R 67 67c 2-iodoacetophenone 2'- bromoacetophenone 99 (S)' 97 (S)' 4'-bromoacetophenone 97 (SI' 3-chloropropiophenone 98 (SI' 4-chloropropiophenone 2,2',4'-trichloroacetophenone 93 (R)d" l-(4-bromophenyl)-4-chlorobutyro- 98 (S)' 98 f phenone OChemical yields of the alcohols were all in the 70-8570 range. Determined by capillary GC analysis of [R]-(+)-a-methoxy-a(trifluoromethy1)phenylacetate. Determined by conversion to the epoxide and measuring the rotation. dSee ref 9. eBy analogy to the reduction of acetophenone and propiophenone: ref 3. 'By analogy to the cyclization of 2-chloroacetophenone.
simple ketones. However, difunctional chiral alcohols would be more valuable for the elaboration of complex compounds. With this in mind, we turned our attention t o more suitably functionalized ketones, in particular, to various halo ketones. Results and Discussion It has been demonstrated that the chiral reduction of 2-haloacetophenones with chirally modified boron reagents
0022-3263/88/1953-2916$01.50/00 1988 American Chemical Societv
Chiral Synthesis via Organoboranes
J. Org. Chem., Vol. 53, No. 13, 1988 2917
is influenced by the nature of the halogen s u b ~ t i t u e n t . ~ ~ ~Table 11. Physical Properties and Absolute Configurations of Tomoxetine and Analogues Thus in the reduction of 2-haloacetophenones with neat Alpine-Borane, the chlorohydrin is obtained in 96% ee, abs the bromohydrin in 93% ee, and the iodohydrin 83% 8e.I compound [ a I z 5 deg ~ , (conc, solv) config mp, "C In order to determine whether similar trends are operating (-)-tomoxetine -43.1 (c 5.9, MeOH) R 160-162 with dIpczBC1,we initiated our present studies with the hydrochloride (+)-tomoxetine +42.9 (c 6, MeOH) S 160-162 chiral reduction of 2-haloacetophenones. The results are hydrochloride summarized in Table I, entries 1-3. However, at this stage -52.0 (c 5, MeOH) S 149-151 (-)-nisoxetine of the investigation, no attempt was made to isolate the hydrochloride compounds (vide infra), but rather to determine the extent (+)-nisoxetine +51.88 (c 4.8, MeOH) R 149-150 and scope of chiral induction by unambiguous analytical hydrochloride methods. -3.04 (c 5.9, MeOH) R 143-144 (-)-fluoxetine hydrochloride -15.52 (C 6, CHCl3) The reduction of acetophenone with dIpczBClgives the (+)-fluoxetine +3.01 (c 5.8, MeOH) S 143-144 S enantiomer pred~minantly.~ The 2-haloacetophenones, hydrochloride +15.83 (C 7.15, CHC13) however, give the R e n a n t i ~ m e r . ~The reduction of 2chloroacetophenone in 95% ee is clearly superior to the plication of this methodology. Tomoxetine [ [R]-(-)-Nresults obtained with the bromide, 80% ee, or the iodide, methyl-y-(2-methylphenoxy)benzenepropaminehydro65% ee. This is not surprising and is in accord with the chloride, Lilly, LY 1396031 is a new drug currently unsteric model proposed for this type of r e d ~ c t i o n .As ~ the dergoing evaluation as a potential antidepressant.'O The steric requirements of the halide increases from chloride (-)-optical isomer has been shown to be nine times more to iodide, the discrimination between RL and Rs decreases, potent than the (+)-isomer. Unlike chemical tricyclic resulting in decreased selectivity in the transition state and antidepressants, e.g., imipramine, (-)-Tomoxetine has been decreased optical induction. On the other hand, placing shown to inhibit specifically norepinephrine uptake in the halide on the aromatic ring, as in 2-bromoacetohumans at doses which are clinically well tolerated and to phenone, would be expected to increase the steric demands be a relatively weak ligand for a-1, a-2, and P-adrenergic of the ring and increase the 70 ee relative to the unsubreceptors.10 The latter receptors are generally regarded stituted compound. This indeed is the case. The reduction as responsible for undesirable side effects associated with of 2-bromoacetophenone with dIpczBClfurnishes the alantidepressants. The patent literature preparation of cohol in 99% ee (Table I, entry 4). However, placing the (-)-pmoxetine involves a long and tedious procedure, bromide in the para position furnishes the corresponding culminating in a highly inefficient resolution (20%) of the alcohol in 97% ee, identical with the parent compound3 racemic mixture.11J2 Clearly an enantioselective prepa(Table I, entry 5). ration of (-)-Tomoxetine would be desirable. We have We next investigated the effect on optical induction by demonstrated that diisopinocampheylchloroborane, varying the position of the halide substituent on the alkyl dIpczBC1,reduces haloaralkyl ketones in a highly enansidechain. Thus, reduction of 3-chloropropiophenone and tioselective and predictable manner, furnishing the [SI4-chlorobutyrophenone gave the corresponding alcohols halo alcohols. We now describe a simple synthesis based in 97% ee and 98% ee, respectively (Table I, entries 6 and on our methodology of (-)-Tomoxetine and (+)-Tomoxe7). From the above examples we conclude that halosubtine, as well as both enantiomers of the cognant comstitution in the a-position of the side chain exerts somepounds, Fluoxetine and Nisoxetine. what greater influence on optical induction than in the (-)-Tomoxetine Hydrochloride (9). [SI-(-)-3ortho position of the aromatic ring. This is further corChloro-1-phenylpropanol is readily available by reduction roborated with 2,2',4'-trichloroacetophenone, which is reof 3-chloropropiophenone with dIpczBC1,in 97% ee, [ a I z 5 D duced in 93% ee (Table I, entry 8). -25.26' ( c 7.05, CHC13),mp 56-57 'C (Table I, 6). HowThese halo alcohols can also be efficiently cyclized. For ever, in the synthesis of biologically active compounds, it instance, the chlorohydrin obtained from the reduction of is desirable to attain enantiomerically pure compounds. 2-chloroacetophenone is cyclized to [R]-phenyloxirane of We have found the simple recrystallization of the 97% ee the same enantiomeric purity as the starting material. [S]-(-)-3-chloro-l-phenylpropanol from hexane serves to Similarly, cyclization of 4-chloro 1-sec-alcohols leads to upgrade the material to essentially 100% ee. Reaction of 2-substituted tetrahydrofurans. Thus, [S]-l-(4-bromoo-cresol in the presence of diethyl azodicarboxylate and phenyl)-4-chlorobutanol furnishes [S]-2-(4-bromotriphenylphosphine (Mitsunobu conditions13) occurred pheny1)tetrahydrofuran of the same enantiomeric purity with complete inversion at the benzylic carbon to furnish as the chloro alcohol. [R]-(-)-l-chloro-3-phenyl-3(2-methy1phenoxy)propane(3), Having demonstrated the utility of chiral reductions [ a I z 5 D -21.7' (c 3.90, CHClJ as an oil in 70% yield. with dIpczBC1,we next turned our attention to the apTreatment of this compound with excess aqueous methylamine in ethanol followed by conversion to the hydrochloride (ethereal hydrogen chloride) gave (-)-To(7) Brown, H. C.; Pai, G. G. J. Org. Chem. 1985,50, 1384. (8) Itsuno, S.; Nakano, M.; Ito, K.; Hirao, A.; Owa, M.; Kanda, N.; moxetine hydrochloride (9), [ a I z 5 D -43.1' (c 5.9 MeOH), Nakahama, S. J. Chem. Soc., Perkin Trans. 1 1985, 2615. mp 158-160 'C (Scheme I). On the basis of our rotation, (9) The R configuration of the product halohydrins is a consequence the compound reported in the literature is only 88% opof the Cahn-Ingold-Prelog convention rather than a change in the mode tically pure.I4 In a completely analogous manner we of attack. synthesized (+)-Tomoxetine starting with [R]-(+)-3-
CH3
k,
(10) Zerbe, R. L.; Rowe, H.; Enas, G. G.; Wong, D.; Farid, N.; Lemberger, L. J.Pharmacol. Exp. Ther. 1985,232, 139. (11)Foster. B. J.: Lavamino, E. R. Eur. Pat. 0 052 492, 1982; Chem. Abstr. 1982,97, 215718d. (12) Molloy, B. B.; Schmiegel,K. K. U. S. Pat. 4018895,1977; Chem. Abstr. 1977, 87, 1345207. (13) Mitsunobu, 0. Synthesis 1981, 1. (14) Reference 12: [ a I z 5-38.1' ~ (c 10,MeOH); mp 165-167 O C .
2918 J. Org. Chem., Vol. 53, No. 13, 1988
Srebnik e t al.
IC, b
OH l
C
-
1
@
2
I d
9,(-1 -Tomoxeti ne
hydrochloride
10. (-)-Fluoxetine hydrochloride
11, (+)-Nisoxetne hybochloride
12, (+)-Tomoxetino
hydrochloride
e
13, (+1- Fluoxet ine
hydrochloride
if
14A-)- Ntsoxetine
hydrochloride
(a) dIpczBCl(the symbol d refers to the reagent derived from (+)-a-pinene; (b) recrystallization; (c) LIpczBCl(the symbol 1 refers to the reagent derived from (-)-a-pinene); (d) o-Cresol, DEAD, Ph3P; (e) p-(trifluoromethyl)cresol, DEAD, Ph,P; (f) Guaiacol, DEAD, Ph3P; (g) aqueous MeNH,, EtOH; (h) ethereal HC1.
our enantioselective synthesis to Nisoxetine hydrochloride, chloro-1-phenylpropanol (Scheme I). we have now correlated the absolute configurations and (-)-Fluoxetine Hydrochloride (10). (&)-N-Methylrotations. The results are summarized in Table 11. y-[4-(trifluoromethy1)phenoxyl benzenepropamine hydrochloride is a widely used and selective inhibitor of serotonin Conclusion uptake15 and has been shown to be chemically effective for the treatment of mental depression.16 In accord with the We have demonstrated that diisopinocampheylchloroproposed mode of action, the (+)-enantiomer was found borane,20dIpc,BC1, is an excellent chiral reducing reagent to be only slightly more active than the (-)-enantiomer. for haloaralkyl ketones. The corresponding halo alcohols In a manner analogous to the preparation of (+)-Tomoxare generally obtained in >go% ee. In certain cases, Le., etine, we treated [S]-3-chloro-l-phenylpropanol with p 3-chloro-l-phenyl-l-propanol, which is initially obtained (trifluoromethy1)cresol to yield [R]-(+)-l-chloro-3in 97% ee, can be upgraded to essentially 100% ee by phenyl-3-[4-(trifluoromethyl)phenoxy]propanol(4),[ L Y I ~ ~ Dsimple recrystallization. In turn, 3-chloro-l-phenyl-l+2.3' (c 10.0, CHCl,). Subsequent treatment with excess propanol provides access to a highly enantioselective methylamine in ethanol, followed by ethereal hydrogen synthesis of the antidepressant agents Tomoxetine, Fluchloride, furnished (-)-Fluoxetine hydrochloride. The oxetine, and Nisoxetine. These halo alcohols can also be physical and optical properties of both enantiomers of cyclized to oxiranes21and 2-substituted tetrahydrofuransz1 Fluoxetine are summarized in Table 11. of the same enantiomeric purities as those of the starting Nisoxetine Hydrochloride (1 1). The o-methoxy alcohols. analogue of Tomoxetine is a potent inhibitor of norExperimental Section epinephrine and serotonin uptake.l' Only recently has (f)-Nisoxetine been resolved into enantiomers and the General Methods. Melting points and boiling points are optical isomers evaluated for norepinephrine uptake inuncorrected. 13CNMR spectra were obtained on a Varian FT-80A spectrometer (20.00 MHz) relative to TMS. GC analysis was done hibition.ls The absolute configuration of the enantiomers, on a Hewlett-Packard 58902 gas chromatograph using a Supelhowever, has heretofore not been determined.lg Applying cowax glass capillary column (15 m) and integrated using a Hewlett-Packard 3390A integrator. Mass spectra were recorded ~
(15) Wong, D. T.; Bymaster, F. P.; Reid, L. R.; Fuller, R. W.; Perry, K. W. Drug Deu. Res. 1985,6, 397. (16) (a) Chovinard, G. A. Clin. J . Psychiatry 1985,46, 32. (b) Stark, P.; Hardison, C. D. Zbid. 1985, 46, 53. (17) Wong, D. T.; Bymaster, F. P. Res. Commun. Chem. Pathol. Pharmacol. 1976, 15, 221. (18)Wong, D. T.; Threlkold, P. G.; Best, K. L.; Bymaster, F. P. J. Pharmacol. Erp. Ther. 1982, 222, 61. However, neither detail of the resolution nor optical properties of the enantiomers have been reported.
(19) Oberlender, R.; Nichols, D. E.; Ramachandran, P. V.; Srebnik, M.
J. Pharm. Pharmacol. 1987, 39, 1055. (20) Both enantiomers of Ipc,BCl are currently available from the
Aldrich Chemical Company; see also ref 6f. (21) Bartok, M.; Lang, K. L. In The Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and Their Sulfur Analogues; Patai, S., Ed.; Wiley: Chichister, 1980; Supplement E, Chapter 14.
Chiral Synthesis via Organoboranes with a Finnigan gas chromatograph-mass spectrometer Model 4000. Elemental analyses were done in house. Optical rotations were recorded on a Rudolph Polarimeter Autopol 111. Reduction of ketones were carried out as described in the literature., Materials. Tetrahydrofuran (Fisher), THF, was distilled from benzophenone ketyl and stored under nitrogen in an ampule. Diethyl ether (Mallinckrodt), ethyl acetate (Mallinckrodt), dichloromethane (Mallinckrodt), pentane (Phillips),hexane (Fisher) were used as received. Anhydrous ethereal hydrogen chloride was prepared from hydrochloric acid and sulfuric acid by using a Brown gasimeter.22 o-Cresol, a,a,a-trifluoro-p-cresol,guaiacol, triphenylphosphine, diethyl azodicarboxylate, and aqueous methylamine were purchased from the Aldrich Chemical Company. Reactions were monitored wherever possible by TLC using Whatman precoated silica plates. Neutral alumina (J. T. Baker & Company, column chromatography) was used for column chromatography. Determination of Enantiomeric Excess. The enantiomeric excess, ee, of the halo alcohols in the initial part of this study was determined by conversion to the MTPA esters, followed by analysis on a methyl silicone column (50 m) or Supelcowax column (15 m). In all cases racemic alcohols gave baseline separations and 1:l ratios of integrated areas. Recrystallization a n d Optical Upgrading of [SI-(-)-3Chloro- 1-phenylpropanol (1). [SI-3-Chloro-1-phenylpropanol (5 g, 29.3 nimol) (1) was dissolved in a minimum amount of hexane and allowed to stand a t 0 "C overnight. The white crystals were filtered and washed with cold pentane to afford 1: yield, 4.25 g, 85%;mp 6 H 2 "C; [aIz3D-26.04 (c 7.00, CHCl,). Analysis of the [R]-(+)-MTPA ester on methyl silicone (50 m, 180 "C) showed one peak. [R I-(-) - 1-Chloro-3-phenyl-3-(2-methy1phenoxy)propane (3). Triphenylphosphine (5.25 g, 20 mmol) and ethyl azodicarboxylate (3.15 mL, 3.48 g, 20 mmol) were added to a solution of [S]-3-chloro-l-phenylpropanol(1) (3.4 g, 20 mmol) and o-cresol (2.06 mL, 2.16 g, 20 mmol) in T H F (50 mL). The mixture was stirred a t room temperature overnight when the reaction was complete (TLC). T H F was removed under aspirator vacuum and the residue triturated with pentane (3 X 50 mL). The combined pentane fractions were concentrated, and the residue was chromatographed on neutral alumina. Elution with pentane and removal of solvent afforded 3.6 g (70%) of the chloro ether 3 as a thick liquid, which was found to be 99% pure by GC: bp 180-200 "C (0.5 mm); [aIz3D-21.7" (c 3.9, CHCI,); 13C NMR 6 150.67, 147.81, 141.28,128.76,127.98,125.32, 122.23,120.96,117.35,112.71, 59.02, 56.07, 41.61; mass spectrum (EI), m / z (relative intensity) 260/262 (1, M+), 224 (1,M - HCl), 153 (21, M - C,H,O), 91 (100, C7H7), (CI) 261 (7.4, M+ H), 153/155 (100, M+ + H - C7H8). [R]-(-)-Tomoxetine Hydrochloride (9). T o the chloro ether 3 (2.6 g, 10 mmol) in a Paar "mini-reactor'' was added aqueous methylamine (40%, 20 mL). Ethanol (10 mL) was added as cosolvent and the solution heated at 130 "C for 3 h. The solution was cooled to room temperature, and the mixture was poured on water (150 mL) and extracted with ether. The ether extract was washed with water and brine and dried (MgS04). HC1 in E E (5 mL of 3.2 M, 16 mmol) was added to the decanted solution, and the solid which separated was crystallized from a mixture of dichloromethane and ethyl acetate: yield, 2.4 g (95%);mp 158-160 "C; [.ID -43.1" (C 5.9, MeOH); 13CNMR 6 155.45, 140.26, 130.85, 128.90, 128.10, 126.95, 126.72, 125.91, 120.91, 113.16,77.08, 46.26, 34.59, 32.87, 16.53; mass spectrum (EI),m / z (relative intensity) 44 (100, CH2NHMe), (CI) 312 (5, M+ + C4H9),256 (100). Anal. Calcd for C17H22C1N0C, 69.98; H, 7.55; C1, 12.18; N, 4.8. Found C, 69.69;H, 7.73; C1, 12.20; N, 4.72. [ S]-(+)-l-Chloro-3-phenyl-3-(2-methylphenoxy)propane (6). This chloro ether was prepared by using the same procedure as for the preparation of 3 using [R]-3-chloro-l-phenylpropanol (2) (3.4 g, 20 mmol), o-cresol (2.06 mL, 20 mmol), triphenylphosphine (5.25 g, 20 mmol), and diethyl azodicarboxylate (3.15 mL, 20 mmol) in T H F (50 mL). Workup gave 6 (3.5 g, 68%) as a thick liquid: bp 180-200 "C (0.5 mm); [ a I z 3 D +21.7" (c 3.9, CHCl,); 13CNMR and the mass spectra were identical with those of 3.
+
(22) Brown, H. C.; Kramer, G . W.; Levy, A. B.;Midland, M. M. Organic Synthesis via Boranes; Wiley-Interscience: New York, 1975.
J. Org. Chem., Vol. 53, No. 13, 1988 2919 [SI-( +)-Tomoxetine Hydrochloride (12). [SI-(+) -Tomoxetine hydrochloride was prepared by using the same procedure as for the preparation of the R-(-)-isomer using the chloro ether 6 and excess aqueous methylamine in a "mini-reactor" a t 130 "C for 3 h. Workup provided 95% [&(+)-Tomoxetine hydrochloride: mp 158-160 "C, +42.9" (c 6, MeOH). All spectral data are identical with those of [R]-(-)-Tomoxetine hydrochloride 9. Anal. Calcd for C17Hz2C1N0C, 69.98; H, 7.55; C1, 12.18; N, 4.8. Found: C, 69.1; H, 7.9; C1, 12.29; N, 4.91. [R]-(+)-l-Chloro-3-phenyl-3-[4-(trifluoromethyl)phenoxylpropane (4). Compound 4 was prepared by using a similar procedure as for the preparation of 3 using [S]-3-chloro-lphenylpropanol (1) (2.57 g, 15 mmol), a,a,a-trifluoro-p-cresol(2.43 g, 15 mmol), triphenylphosphine (3.93 g, 14 mmol), and diethyl azodicarboxylate (2.36 mL, 15 mmol) in T H F (40 mL) a t room temperature. Workup gave 4 as a thick liquid: bp 180-200 "C (0.5 mm); [aIz3D +2.3" (c 10, CHCl,); 13CNMR (CDCl,) 6 140.50, 129.39,128.66,127.56, 127.38, 127.19,127.01,126.34, 116.45,77.82, 41.69,41.38; mass spectrum (EI), m/z (relative intensity) 153/155 (45),91 (loo), (CI) 314/316, (1,M'), 153/155 (100).Anal. Calcd for Cl6HI4C1NO: C, 61.05; H , 4.45; C1, 11.29; F, 18.12. Found: C, 61.06; H, 4.51; C1, 11.16; F, 18.22. [R 1-(-)-Fluoxetine Hydrochloride ( 10). [R ]- (-) -Fluoxetine hydrochloride was prepared by using a similar procedure as was used for [R]-(-)-Tomoxetine hydrochloride utilizing the chloro ether 4 (1.57 g, 5 mmol) and excess aqueous methylamine in ethanol as cosolvent in a "mini-reactor'' at 130 "C for 3 h. Workup provided 1.55 g, 90%, of the recrystallized (CH2Clz/EtOAc) Fluoxetine hydrochloride: mp 142-143 "C; [ a I z 2 D -3.01" (c 5.3, MeOH); -15.52" (c 7.15, CHC1,); 13C NMR (CDCl,) 6 160.10, 139.46, 129.28,128.66,127.35,127.16,126.96, 126.77, 126.08, 116.26, 55.49,46.32, 34.77, 33.13; mass spectrum (EI),m / z (relative intensity) 44 (100, CH,NHMe), (CI) 310 (100, M+ + H), 148 (12). Anal. Calcd for C17HlgClF,NO: C, 59.05; H, 5.54; N, 4.05; F, 16.48; C1, 10.25. Found: C, 59.02; H, 5.6; N, 4.13; F, 16.67; C1, 10.5. [S]-(-)-l-Chloro-3-phenyl-3-[4-(trifluoromethyl)phenoxylpropane (7). The chloro ether 7 was prepared with [R](+)-3-chloro-l-phenylpropanol(2)(2.56 g, 15 mmol), a,a,a-trifluoro-p-cresol (2.43 g, 15 mmol), triphenylphosphine (3.93 g, 15 mmol), and diethyl azodicarboxylate (2.36 mL, 15 mmol) in T H F (40 mL) at room temperature. Workup gave 7 as a thick liquid: yield, 3.07 g, 65%; bp 180-200 "c (0.5 mm); [ a ] D -2.2" (c 12.5, CHCl,); 13CNMR and mass spectrum were identical with those of 4. [SI-(+)-Fluoxetine Hydrochloride 13. [SI-(+)-Fluoxetine hydrochloride was prepared by a similar procedure as was used for 10, utilizing the chloro ether 7 (1.59 g, 5 mmol) and excess aqueous methylamine in ethanol as cosolvent in a "mini-reactor" at 130 "C for 3 h. Workup provided 1.55 g (93%) of the recrystallized (CHzClz/EtOAc) 13,mp 142-143 "C; [ a I z 2 D +3.04" (c 5.9, MeOH), [.Iz2D +15.83" ( c 6, CHCI,); I3C NMR (CDCl,) 6 160.08,139.44,129.26, 128.64,127.30,127.12,126.76,116.25,77.48, 46.32; 34.77, 33.14; mass spectrum (EI), m / z (relative intensity) 44 (100, CHzNHMe), (CI) 310 (100, M+ + H), 148 (12). Anal. Calcd for Cl7HIgC1F,NO: C, 59.05; H , 5.54; N, 4.05; F, 16.48; C1, 10.25. Found: C, 58.70; H, 5.58; N, 4.29; F, 16.38; C1, 10.35. [It]-( +)-l-Chloro-3-phenyl-3-(2-methoxyphenoxy)propane (5). The chloro ether 5 was prepared by a procedure similar to the one used for the preparation of 3 using [S]-(-)-3-chloro-lphenylpropanol (1) (1.71 g, 10 mmol), guaiacol (1.1mL, 10 mmol), Ph,P (2.62 g, 10 mmol), and diethyl azodicarboxylate (1.57 mL, 10 mmol) in T H F (40 mL) at room temperature. Workup and chromatography with neutral alumina (hexane/ethyl acetate, 97:3) gave 1.7 g, 62%, of 5 as a thick liquid, bp 180-200 "C (0.5 mm). The liquid, upon cooling, solidified and could be recrystallized from pentane: mp 59-61 "C; [ a I z 3 D +40.96' ( c 7.8, CHC1,); 13C NMR (CDC13) 6 150.67, 147.81, 141.28, 128.76, 127.98, 126.32, 122.23, 120.96, 117.35, 112.71, 79.02, 56.07, 41.61; mass spectrum (EI), 276/278 (1, M+), 240 (M+ - HCl), 260/262 (M - CHJ, 91 (loo), 124 (82), (CI) 277/279 (13.6, M+ + H), 153/155 (100). Anal. Calcd for Cl7HZ2C1NO2:C, 69.44; H, 6.15; C1, 12.84. Found: C, 69.67; H, 6.3; C1, 12.65. [R]-(+)-Nisoxetine Hydrochloride (11). [R]-(+)-Nisoxetine hydrochloride was prepared by a procedure similar to the one used for 4, using the chloro ether 5 (1.35 g, 5 mmol) and excess aqueous methylamine in ethanol (1mL) in a "mini-reactor" a t 130 "C for
J . Org. Chem. 1988, 53, 2920-2925
2920
3 h. Workup gave 1.41 g (91%) of recrystallized (CH2C1,/EtOAc) 11: mp 149-150 OC; [ a ] D +51.88' (c 4.8, MeOH); l3C NMR 6 150.00, 140.18, 129.29,129.07,128.58,126.14,123.03, 121.32, 117.29, 112.23,81.83, 56.44, 47.56, 34.20, 33.19; mass spectrum (EI), m / z (relative intensity) 167,44 (loo), 148 (8), (CI) 272 (100, M'). Anal. Calcd for C1,HzzC1NO2: C, 66.34; H, 7.15; N, 4.55; C1, 11.5. Found: C, 66.08; H, 5.29; N, 4.66; C1, 11.59. [S 1- (-)-Chloro-3-phenyl-3-(2-methoxyphenoxy )propane (8). Chloro ether 8 was prepared by a procedure similar to the one used for 5: yield, 1.64 g (60%); mp 59-61 "C; -41.6' (c 3, CHCI,); 13C NMR and mass spectrum were identical with those of 5. [SI-(-)-Nisoxetine Hydrochloride (14). [SI-(-)-Nisoxetine hydrochloride was prepared by a procedure similar to the one used for 11: yield, 1.41 g (91%); mp 149-151 OC; [ a I z 3 D -52' ( c 5, MeOH); 13C NMR and mass spectra were identical with those of 11.
Acknowledgment. We would like t o thank the United States Army Research Office ( D U G 29-82-K-0047) for the
financial assistance which made o u r studies possible.
Registry No. 1, 100306-34-1;2, 100306-33-0;3, 114446-47-8; 4, 114446-48-9;5 , 114446-49-0;6, 114446-50-3;7, 114446-51-4;8, 114446-52-5;9, 82248-59-7; 10, 114247-09-5;11, 114446-53-6;12,
82857-39-4; 13, 114247-06-2;14,114446-54-7;dIpc,BC1, 85116-37-6; 'Ipc2BC1, 112246-73-8;4'-bromc-4-chlorobutyrophenone, 4559-96-0; 2-~hloroacetophenone,532-27-4;2-bromoacetophenone, 70-11-1; 2-iodoacetophenone, 4636-16-2;2'-bromoacetophenone, 2142-69-0; 4'-bromoacetophenone, 99-90-1; 3-~hloropropiophenone,936-59-4; 4-~hloropropiophenone,ti285-05-8; 2,2',4'-trichloroacetophenone, 4252-78-2; [R]-2-chloro-l-phenylethanol, 56751-12-3; [R]-2bromo-1-phenylethanol, 73908-23-3; [R]-2-iodo-l-phenylethanol, 85611-59-2; [S]-l-(2-bromophenyl)ethanol, 114446-55-8; [SI-1(4-bromophenyl)ethanol, 100760-04-1; [S]-2-(4-bromophenyl)tetrahydrofuran, 114446-56-9; [S]-4-chloro-l-phenylbutanol, 65488-06-4; [R]-l-(2,4-dichlorophenyl)-2-chloroethanol, 114446114446-58-1; [ R ] 57-0; [S]-l-(4-bromophenyl)-4-chlorobutanol, phenyloxirane, 20780-53-4; o-cresol, 95-48-7; a,a,a-trifluoro-pcresol, 402-45-9; guaiacol, 90-05-1.
Relative Reactivities of Acetals and Orthoesters in Lewis Acid Catalyzed Reactions with Vinyl Ethers. A Systematic Investigation of the Synthetic Potential of Acetals and Orthoesters in Electrophilic Alkoxyalkylations of Enol Ethers Uwe von der Bruggen, Roswitha Lammers, a n d H e r b e r t Mayr* Institut fur Chemie der Medizinischen Uniuersitat zu Lubeck, Ratzeburger Allee 160, 0-2400 Lubeck, Federal Republic of Germany
Received J a n u a r y 4, 1988 The relative reactivities of acetals and orthoesters in BF3.0Etz-catalyzed reactions with methyl vinyl ether (-78 "C, CH2C12)have been determined by competition experiments. A reactivity increase by 5 orders of magnitude was found in the series: saturated acetals < methyl orthoformate < benzaldehyde acetals < a$-unsaturated acetals; formaldehyde acetals as well as orthoacetates and orthobenzoates did not react under these conditions. The Krel values of the para-substituted benzaldehyde acetals follow a Hammett u correlation ( p = -4.6). Whereas the krel values of the aldehyde acetals are correlated with the corresponding rate constants of acid-catalyzed hydrolyses, ketals and orthoesters deviate from this correlation. It is concluded that the krellisting in Scheme I1 can be used to predict the results of Lewis acid catalyzed additions of acetals and orthoesters toward vinyl ethers: The formation of 1:l addition products may only be expected, if the relevant functional group of the reactants is listed below the functional group of the potential 1:l products. Muller-Cunradi and Pieroh discovered i n 1939 that acetals react with enol ethers i n the presence of a Lewis acid t o give 3-a1koxyacetals.l
T h i s reaction, which was
later suggested to proceed via carbocationic intermediates,2 has become an i m p o r t a n t m e t h o d i n organic s y n t h e ~ i s . ~ Isler's carotine synthesis, for example, employs additions of unsaturated acetals t o ethyl vinyl ether and ethyl propenyl ethers as key steps for the construction of t h e polyene fragment.4 Hoaglin a n d Hirsch2 have already recognized t h a t reaction 1 is n o t generally appplicable for the synthesis of ( 1 ) Muller-Cunradi, M.; Pieroh. K. U.S.Patent 2 165962; Chem. Abstr. 1939, 33, 8210. (2) Hoaglin, R. I.; Hirsch, D. H. J. Am. Chem. SOC.1949, 71, 3468. (3) Reviews: (a) H. Meerwein In Houben-Wed, Methoden der Organischen Chemie; Thieme: Stuttgart, 1965, Vbl. VI-3, pp 199. (b) Effenberger, F. Angew. Chem. 1969,81,374; Angew. Chem., Int. Ed. Engl. 1969,8, 295. (c) Povarov, L. S. Russ. Chem. Reo. (Engl. Transl.) 1965, 34, 639. (d) Mathieu, J.; Weill-Raynal, J. Formation of C-C Bonds; Thieme: Stuttgart, 1979; Vol. 111,pp. 196. (e) Makin, S.M. Russ. Chem. Rev. (Engl. Transl.) 1969, 38, 237. (0 Makin, S. M. Pure Appl. Chem. 1976, 47, 173. (4) (a) Isler, 0.;Lindlar, H.; Montavon, M.; Ruegg, R.; Zeller, P. Helu. Chim. Acta 1956, 39, 249. (b) Isler, 0. Angew. Chem. 1956, 68, 547.
1:l addition products, since the adducts 3 m a y add t o the double b o n d of 2 i n a similar m a n n e r as 1, thus leading t o the formation of higher adducts. As t h e yield of t h e
4'C(OR), R't
R'
OR t
H2C=C/
'R
'
a arid
OR I
S2-I!--Ch,--C--OR I I OR R'
(1)
1:l adducts 3 depends o n the relative reactivity of t h e acetals 1 a n d 3 toward vinyl ethers, several papers were addressed t o t h e relationship between structure a n d reactivity of acetal^.^ In a n excellent review, Povarov has interpreted the results of acetal and orthoester additions t o enol ethers i n terms of relative reactivities of reactants and products using t h e qualitative reactivity sequence: saturated acetals < aromatic acetals = ortho esters < a,(5) (a) Yanovskaya, L. A. Izu. Akad. Nauk SSSR, Ser. Khim. 1965, 1638; Chem. Abstr. 1966, 64, 33426. (b) Yanovskaya, L. A.; Kucherov, V. F. Izu. Akad. Nauk. SSSR, Ser. Khim. 1965, 1657; Chem. Abstr. 1966, 64, 1 9 4 7 ~ .(c) Fueno, T.; Okuyama, T.; Furukawa, J. J . Polym. Sci., Part A-1 1969, 7 , 3219.
0022-3263/88/1953-2920$01.50/00 1988 American Chemical Society
